1. Field of the Invention
This invention relates to the structural loading of materials having large differences in their moduli of elasticity. It particularly relates to panels comprising a composite system of steel bands laminated into fiberglass-polyester sheets. It further relates to butt joints for conjoining and interconnecting such panels. It additionally relates to modular construction of storage tanks by conjoining fiberglass panels with creep-resistant joints therebetween.
2. Review of the Prior Art
Storage tanks are very common types of equipment in the chemical and allied industries for accumulating reserve stocks of raw materials against a disturbance in regular deliveries or because shipment is periodical and in large quantity. In addition, storage tanks are used for intermediate storage in a process or for storage of a product. If a tank serves as one of the steps in a manufacturing process, it is designated a process tank, such as dissolving tanks, blending tanks, and feeding tanks, and may include other functions such as thickening, settling, clarifying, and fermenting. Tanks built at grade are of flat-bottom type and may rest on a foundation of sand or on both sand and a curbing of concrete slightly larger than the tank diameter.
In addition to such liquids as water, brine, acid solutions, caustic solutions, wood pulps, mining pulps, petroleum, naphtha, gasoline, and drilling muds that are commonly stored in storage and process tanks, flowable solids can also be stored therein and can similarly generate hydrostatic heads. Such flowable solids include grains, flour, powdered coal, fertilizer, powdered sulfur, sugar, salt, and various other powdered and particulate products.
Most tanks are made of steel, but corrosion-resistant tanks have frequently been made from thin-walled composite materials such as a composition containing a phenolformaldehyde resin and an acid-digested asbestos fiber or graphite. Such tanks resist the action of any acid and of many salt solutions. They are tough, strong, and impact resistant and resist sustained temperatures of 265.degree. F. (130.degree. C.).
During recent years, composite panels constructed of fiberglass and polyester resin have become widely used and highly preferred. Fiberglass-polyester resin has been used in the manufacture and installation of storage tanks because of its excellent resistance to a wide range of chemicals, good impact strength, and light weight.
Unfortunately, random-chopped fiberglass-resin composite panels have a tendency to creep under heavy loads over long periods of time. This slow movement creates stress concentrations, particularly near the bottom sides and at the joints or butted edges of a tank under load. Because of the concentrated loads at such places, there is a distinct possibility of eventual failure of the plastic structure. Furthermore, it is known that when reinforced polyester and epoxy resins are placed in contact with liquids over a long period of time, there is a gradual reduction in tensile strength. Designers are forced, by these characteristics of the reinforced panels that are currently in the market, to design and fabricate under considerable uncertainty and with exaggerated safety features.
Non-pressurized storage and process tanks create unique design problems because of uneven loading of the walls and the wall intercepts in accordance with the pressure created by the head of the liquid therein. When a tank is filled with liquid, there is no pressure at the top and maximum pressure at the bottom along the sides of the tank. Consequently, stretching along the walls is greatest near the bottom and makes the bottom circumference greater than the circumference of the base or of the top of the tank. Because of such concentrated stretching and bending, stress concentrations are also greatest near the bottom of the tank, and higher stress means accelerated corrosion, as indicated in FIGS. 1 and 2 of the drawings.
Another design factor arises because hydrostatic pressure tends to force the sides of a tank to assume a prefectly round shape. Therefore, any configuration, such as a field-assembled joint, that is not aligned along the circumference of a circle creates concentrated bending and stretching.
All joints under loads will generate stress concentrations and thus accelerate corrosion, especially in field-erected tanks constructed from thin-walled, creep-susceptible panels. During modular erection of a tank on site, the curved panels are generally assembled with a lap joint, a bolted butt joint, or a welded butt joint. As the walls of the tank increase in diameter because of stretching by hydrostatic pressure, the bends straddling a bolted butt joint particularly tend to become straightened out by the pressure exerted along the walls and become loci of accelerated corrosion, as indicated in FIG. 3. Furthermore, added fixtures in a tank wall, such as a valve or hatch, create stress concentrations at the opening in the wall because the opening can bear no load and must transmit its share of the load to the adjoining wall above and beneath its sides, as indicated in FIG. 4.
It is recognized that any material stretches at a rate defined by its modulus of elasticity. A fiberglass-resin composite of randomly oriented short fiber construction stretches approximately 30 times per unit area more than steel under any given load. Steel and fiberglass bonded in a plastic composite will stretch at a rate that is defined by their composite modulus of elasticity, i.e., whether the force is considered to be exerted on the steel or on the composite or on both, the movement will be the same.